For example, an X-ray metal grating structure for receiving X-rays is utilized in various devices, as an element having a large number of parallel periodic structures, and, in recent years, its application to X-ray imaging devices has been attempted. In the field of X-ray imaging devices, from a viewpoint of reduction in exposure dose, great interest has been recently shown in X-ray phase imaging, which is based, for example, on a Talbot interferometer or a Talbot-Lau interferometer. In an X-ray imaging device employing the Talbot-Lau interferometer, three X-ray metal grating structures consisting of a zeroth grating, a first grating and a second grating are used. The zeroth grating is a normal grating utilizable to modify a single X-ray source to a multiple source, i.e., is capable of dividing a flux of X-rays radiated from the single X-ray source, into a plurality of fluxes of X-rays (plurality of X-ray beams) and radiating them therefrom. The first and second gratings are diffraction gratings arranged in such a manner as to be spaced apart from each other by a Talbot distance, and make up the Talbot-Lau interferometer (or Talbot interferometer). In terms of a diffraction process, the diffraction grating can be classified into a transmissive diffraction grating and a reflective diffraction grating, wherein the transmissive diffraction grating includes an amplitude-type diffraction grating (absorptive diffraction grating) light-transmissive substrate, and a phase-type diffraction grating in which a plurality of optical phase-shifting portions are periodically arranged on a light-transmissive substrate.
Such X-ray phase imaging requires an absorptive diffraction grating capable of providing clear contrast between transmission and non-transmission of X-rays having high penetration property, and a phase-type diffraction grating capable of providing clear phase contrast (shift) of X-rays. Thus, there is a need for a high-aspect ratio-structured grating having an extremely high aspect ratio, for example, of 3 or more. For this purpose, there has been proposed a fabrication method using silicon processing created by applying semiconductor processing techniques. As an example, WO 2012/008118A (Literature 1) discloses a production method for a metal grating structure. The metal grating structure production method disclosed in this Literature 1 comprises forming a recess (slit) using a dry etching apparatus, and then burying a metal in the recess.
Meanwhile, the dry etching apparatus is costlier than a wet etching apparatus, assuming that the two apparatuses process isometric workpieces. Thus, in the method disclosed in the Literature 1, the use of the costly dry etching apparatus inevitably leads to an increase in production cost. Particularly, in the case where a large-area substrate having a size equal to or greater than that of an 8-inch wafer is subjected to dry etching, its production cost becomes higher.
Therefore, for performing the processing at lower cost, it is conceivable to utilize a wet etching process. However, in the case where a recess is formed in a substrate by a commonly-used wet etching process, dissolution progresses not only in a depth direction but also in a lateral direction, with respect to an opening of a resist, because a dissolving action of an etching solution is isotropic. For this reason, in the wet etching process, a so-called undercut occurs. As a result, the recess formed by the wet etching process has a side surface inclined with respect to a principal surface of the substrate. Thus, a conventional processing based on the wet etching process has difficulty in forming a recess having a side surface perpendicular to the principal surface.